This invention relates to automobile traveler services, using a combination of the global positioning system (GPS) and the commercial cellular telephone network and provides the additional system capability to passively monitor local AM radio stations, within urban canyon areas, and use these signals to augment GPS tracking when automobile receipt of GPS signals is impaired by the urban environment. This enhanced system may be termed No-Outage GPS/AM Positioning System and incorporates the following unique features:
1. International system, with no coverage gaps; this reflects the global coverage provided by GPS and the international standardization of the AM radio waveform.
2. Reception and processing of GPS and AM radio signals, without any interaction or coordination with the GPS and AM systems.
3. GPS is the primary positioning system, with AM signals used for augmentation only in urban canyon areas where GPS is least reliable but AM is most reliable.
4. Each urban canyon area of interest contains a low-cost reference station that measures "AM differential corrections" and provides them to the vehicles via the cellular network.
5. Position accuracy .sup..about. 30m (3.sigma.); this reflects differential GPS and also reflects the accuracy achievable via tone ranging of AM signals (the AM wavelength ranges from .sup..about. 600' to 2000').
6. Within the urban canyon environment, AM signal propagation reflects a degree of robustness against multipath, relative to higher frequency radio signals (e.g., FM), due to the long wavelength of AM signals.